Because this blocker only has 5 toughness and the attacker has trample, the excess 3 damage is dealt to the defending player
He has one of the longest, twistiest tales the game has to offer, starting way back in 1993s Legends
While it technically gives your opponent extra colorless mana, would you rather your opponent had an extra land or a Gideon, Ally of Zendikar
Weve got millions of minutes of video of lightning, just ask anyone in Tampa, Florida and theyll show you the hours of video theyve recorded from their porch
But when you tack on milling synergies in its first chapter, returning all lands from your graveyard to the battlefield in the second chapter and another target creature getting +X/+X equal to the number of lands you control in the final chapter, youve got the makings of a top-flight game-finisher and yet another example of Summons being playgrounds for the Timmys/Tammys of the world (like me)
Very happy they still graded it